Field of the Invention
This invention relates generally to radioisotope imaging apparatus and more particularly to a novel collimator kit for providing collimators with different performance characteristics for radioisotope imaging apparatus.
Prior Art
The in-vivo imaging of organs, embolisms, tumors, etc., through the use of diagnostic radioisotopes is now a practice routinely performed in hospitals of every size in every locale. The widespread acceptance of isotope imaging is a product of the medically and the surgically proven diagnostic reliability afforded by isotope scanning and the ease with which organs can be visutlized. There is no pain or morbidity associated with an isotope scan. A technician or nurse can both prepare the patient and perform the scan with a minimum of training. In many situations, isotopic scanning provides more accurate diagnostic information earlier and with less trauma to the patient than conventional methods.
Radioisotope scans give accurate, positive detection of brain lesions and hermatomas without the hazards of other, less direct techniques. Scanning affords early detection of pulmonary emboli unrecognizable by other techniques. Renal scans localize peripheral leisions and regions of abnormal function quickly without pain or morbidity. Thyroid scans distinguish the "cold" non-functioning nodule, which may be malignant, from the "hot" nodule, which is seldom malignant, and locate thyroid metastases. The presence and size of pericardial effusions and even myocardial infarcts have been demonstrated by heart scanning. Liver scans reveal parasitic invasion, metastatic lesions, subdiaphragmatic abscesses and the extent of hepatic cirrhosis.
Splenic size abscesses or trauma damage may be accurately assessed without surgery, as may pancreatic carcinoma.
Radiopharmaceuticals utilized for the above mentioned studies are readily available. The radiation dose administered to the patient is less than that received during most X-ray examinations. Training and licensing procedures require minimum time.
The procedures and equipment involved in radioisotope imaging are well known to those versed in the art and hence need not be explained in elaborate detail. Suffice it to say that the imaging procedure involves administering to the patient a radiopharmaceutical, that is a pharmaceutical containing a radioactive isotope, which tends to migrate through the body to, and accumulate in the body portion or organ to be examined. A scanning head is then moved back and forth along a series of parallel scan lines over the corresponding surface area of the body to detect the radiation emanating from the body. The output of the scanning head actuates a radiation counting and recording instrument which produces a visual recording or picture, referred to as a "scan," depicting the body portion or organ being examined in terms of variations in radiation intensity along the scan lines. This recording or scan may be presented either in black and white or in color and either on a television screen or on paper.
The scanning head of a radioisotope imaging instrument has two primary elements which are a detector and a collimator. The detector is the actual radiation sensor of the head. The collimator is situated in front of the detector and effectively serves as a radiation "lens" which "focuses" the detector on a relatively small area of the patient's body in such a way that the detector "sees" and receives radiation from this area only. The primary object of the present invention is to improve this collimator.
The theory, construction, and operation of radioisotope imaging collimators are well understood by those versed in the radioisotope imaging art. Accordingly, it is unnecessary to present an elaborate explanation of such collimators, Suffice it to say that a radioisotope collimator, in its current form, consists of a cylindrical body constructed of a material which is relatively opaque to the gamma radiation from the radiopharmaceuticals employed for radioisotope imaging, Extending endwise through this body are a multiplicity of conically tapered collimating holes. The small ends of these holes open through the front end face, referred to herein as the entrance face, of the body to form an array of inlet pupils. The large ends of the collimating holes open through the rear end face, referred to herein as the exit face, of the body to form an array of exit pupils.
The collimating holes are arranged in a regular geometric pattern over the major cross-section of the collimator body with a generally uniform center spacing between the holes in any given cross-sectional plane of the body. All of the holes, except that hole, if any, which extends along the central longitudinal axes of the body, are inclined at acute angles to the body axis in such a way that the longitudinal axes of all the holes intersect the body axis substantially at a common point (focal point) located a given distance (focal distance) beyond the inlet body face. The plane which passes through the focal point normal to the body axis is the focal plane of the collimator.
Two different loci may be ascribed to each collimator hole. The first of these loci is that generated by rotating about the longitudinal axes of the hole a line located in a plane containing the hole axes and lying on the wall of the hole. The second locus is that generated by rotating about the hole axis a line intersecting the axis and contacting diametrically opposite points along the edges of the inlet and exit pupils of the hole. In the radioisotope imaging field, the region bounded by the first locus of each collimator hole is referred to as a full response region. The region outside of the full response region and bounded by the second locus of the hole is referred to as a partial response region. The entire exit pupil area of each collimator hole is visible from every point in its full response region. Within the partial response region, on the other hand, only a portion of the exit pupil area of a collimator hole is visible, the visible pupil area diminishing as the distance from the hole axis increases. The overall field of view of each collimator exit pupil is thus represented by the region bounded by the second locus of the corresponding collimator hole.
The intersection of the field of view of each exit pupil of a radioisotope imaging collimator with every plane normal to the collimator body axis is substantially a circular area. The collimator focal plane is unique in that in this plane, and in this plane only, these intersection areas of all the exit pupil fields of view are super-imposed. For convenience, the circular area of imposition of the fields of view of the several exit pupils in the focal plane is hereafter referred to as the resolution field of the collimator. Thus, the collimator has a resolution field equal to the field of view of a single collimator hole at the focal plane. In every other plane normal to the collimator body axis, the circular areas of intersection of the exit pupil fields of view with the plane are displaced or offset relative to one another.
In use, a radioisotope imaging collimator is installed in the scanning head of a radioisotope imaging instrument in a position directly in front of the radiation detector with the rear exit face of the collimator facing the detector. Assuming the collimator body to be totally opaque to gamma radiation, which it is not, during scanning movement of the head over a patient's body, the detector receives only that radiation emanating from the patient which passes through the collimator holes. Radiation sources which are located in the focal plane of the collimator within its resolution field appear to the detector to be sharply defined. Radiation sources located in the field of view of the collimator but away from its focal plane appear blurred to the detector. In other words, the collimator effectively focuses the detector on the region of the patient's body located in the focal plane of the collimator within its resolution field.
The scanning head of the imaging instrument is adjusted toward or away from the patient's body to locate focal plane of the collimator at the depth of the body region or organ to be examined. During scanning motion of the head over the body, therefore, the resolution field of the collimator scans back and forth across the body region or organ. The instrument then records a scan representing the body region or organ in terms of the varying radiation intensity along the scan lines of the head.
A radioisotope imaging collimator has three primary characteristics or parameters, collectively referred to herein as performance characteristics, which determine its suitability for various types of radioisotope imaging purposes. These performance characteristics are resolution, depth response, and sensitivity. Resolution refers to the size of the collimator resolution field and is determined by the size of the collimator holes. The smaller this field, the finer the collimator resolution and the larger the field, the coarser the resolution. Depth response refers to the spacing (focal distance) between the entrance face of the collimator body and its focal plane. Sensitivity refers to the effective radiation counting rate attainable with the collimator from a given radiation source and is determined in part by the size and in part by the number of collimator holes.
The optimum collimator performance characteristics for any given radioisotope imaging application are well known to those versed in the art. Accordingly, it is unnecessary to discuss this matter in the present disclosure. Suffice it to say that the currently available collimators suffer from the disadvantages that each has fixed performance characteristics and is thus suitable for only one or at most only a few different imaging applications. As a consequence, each radioisotope imaging instrument must be equipped with a set of perhaps seven, eight or more heads having collimators with different performance characteristics at a cost of $600 - $800 each.